This invention relates to the field of radio frequency amplifiers for hand-held communication devices, using deep submicron CMOS technology (e.g. 0.25 xcexcm).
Amplifiers for radio frequency transmission generally consist of a driver or preamplifier stage and a power amplifier stage. In the design of high voltage analog CMOS amplifiers, the potential for gate rupture limits the supply voltage and thus, the power that may be provided by a particular amplifier design. MOSFET devices for radio frequency power amplifiers are being designed for operations at very low supply voltages (e.g. less than 2.5 V) in hand-held devices to prevent gate rupture. However, decreasing the supply voltage for a given technology decreases the output power and efficiency of the amplifier.
Shaping the voltage waveform of the amplifier input signal has been proposed as a potential solution to increasing supply voltage without gate rupture in xe2x80x9cRF Power Amplifiers for Wireless Communicationsxe2x80x9d by Steve Cripps. Others have proposed lightly doping the drain regions and increasing the oxide thickness to increase the breakdown voltage for MOS devices.
The above references are hereby incorporated herein in whole by reference.
Deep submicron CMOS technology (e.g. 0.25 xcexcm) is being developed for achieving high cut-off frequencies required for higher frequency hand-held communication devices. Such technology provides very thin gate oxide layers (e.g. 50 A for 0.25 xcexcm devices) which results in breakdown at peak voltages of around 6V between the gate and the drain of the power amplifier. In radio frequency analog CMOS amplifiers the signal swing at the drain of the power transistor may be 2-3.5 times the supply voltage.
In a first aspect of the invention herein, the driver portion of the amplifier modifies the shape of the driver output signal with respect to the driver input signal. Specifically the driver increases the amplitude of the positive excursions with respect to the amplitude of the negative excursions with respect to the average voltage of the signal (the DC component with respect to ground). Preferably, the peak voltage of the positive excursions with respect to average voltage (the DC component with respect to ground) is at least 30% less than the peak voltage of the negative excursions with respect to the average voltage. More preferably, the negative excursions are less than half the positive excursions.
In a second aspect of the invention, the input signal to the power amplifier stage has a sufficient positive bias to reduce the peak voltage difference between the terminals of the power transistor so that breakdown is prevented. Preferably, the bias voltage is equal to at least 50% of the amplitude of the negative excursions with respect to the average voltage of the driver output signal and more preferably is approximately equal to the amplitude of the negative excursions. Preferably, the signal shape modification together with the signal bias is selected so that the voltage of the peak negative excursions have an absolute value with respect to ground that is less than 30% of the positive value of the positive excursions with respect to ground, and more preferably, less than 10% of the positive value of the positive excursions. By keeping the lower excursion close to zero volts at the gate of the drive transistor of the power amplifier, the maximum supply voltage of around 2.5V, typically used for 0.25 xcexcm CMOS, can be provided to the power amplifier without gate rupture.
Herein ground simply means a reference voltage that is different than the supply voltage. For handheld devices, the ground voltage level is different than the voltage level of the Earth.
In a third aspect of the invention the driver includes an inverter to both modify the shape of the driver output signal with respect to the driver input signal and to provide the required bias.
In a fourth aspect of the invention, the amplifier driver includes a CMOS inverter with a pull-down inductance and a pull-up inductance. The pull-down and pull-up inductances as well as the relative size of a PMOS and NMOS transistors of the inverter are selected to both modify the shape of the driver output and to provide the required bias.
Advantageously, the bias point and hence the output power of the power amplifier can be regulated by controlling the supply voltage to the driver stage of the amplifier. Alternately, the output power can be regulated by controlling the supply voltage to the power amplifier stage of the amplifier. It is also possible to control the output power by controlling the amplitude of the input signal of the driver stage.